The present invention relates generally to a system for producing a semiconductor and in particular to a method and apparatus for semiconductor filming wherein a thin film, such as an oxidation film, is deposited on a surface of a silicon wafer.
Semiconductor manufacture involves a step wherein an oxidation film is generated as an insulation film on a wafer surface, which step is carried out by means of an oxidation system. A known oxidation system will be explained hereunder with reference to FIG. 3.
Inside a reaction tube 1, there is provided a boat 3 carrying a lot of horizontally oriented wafers 2 laid in a multi-storied fashion. The boat 3 is vertically disposed, via a boat stand 5, on a furnace opening cover 4 which airtightly covers a lower end of the reaction tube 1. In a ceiling of the reaction tube 1, there are provided a plurality of gas introduction holes 6 communicating with a gas introduction pipe 7. An exhaust pipe 8 communicates with the inside of the reaction tube 1 at a lower part thereof. The gas introduction pipe 7 is connected to a source of oxygen gas not shown and has a valve 9 and a flow rate adjuster 10 thereabove.
Referring to FIG. 4, oxidation operation in the known system will now be explained.
After the wafer 2 are loaded on the boat 3, the boat 3 is introduced into the reaction tube 1. The internal temperature of the reaction tube 1 is elevated at a rate of 5-10.degree. C./min to a preset temperature (900.degree. C. in FIG. 4) and maintained at that temperature. The exhaust pipe 8 is opened to feed oxygen from the gas introduction pipe 7 into the reaction tube 1 to thereby oxidize the wafer 2 surface. Upon completion of the oxidation process, the gas is exhausted from the exhaust pipe 8.
After the oxidation process is completed, the internal temperature is lowered at a rate of 2-5.degree. C./min to a predetermined temperature, e.g., 800.degree. C. and maintained at that temperature. After lapsing of a predetermined time period, the boat 3 is taken out of the reaction tube 1.
Since the exhaust gas is eventually exhausted to the atmosphere, the back pressure of the exhaust gas is greatly influenced by the atmosphere. Thus, internal pressure of the reaction tube 1 is liable to be influenced by the atmospheric pressure. Further, because the oxidation rate of the wafer depends on the partial pressure of oxygen, a change in the atmospheric pressure leads to a change in the partial pressure, and the oxidation rate changes as the atmospheric pressure changes. Consequently, there has been a problem that the quality of the oxidation process varies depending on the atmospheric pressure during such process and thus becomes unstable.